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Day 23

英文重现

When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18.

——GMAT OG 2015, Reading Comprehension, Passage 15

解析

1.现在分词可以充当状语，表示主句动作发生后造成的结果，比如“He fired, killing one of the birds on the tree”，打死了树上的一只鸟就是开枪的后果。第一句中的“steadily reducing…return to it”也是充当状语，说明冰原变大后的结果；“evaporated from the ocean”则是过去分词作后置定语，与后面的从句“that will eventually return to it”一起修饰前面的名词“water”（“it”指代“ocean”）。

2.第二个句子中，“when”引导的时间状语从句“when water evaporates from the ocean surfaces”修饰“tend”；主句中的词组“become enriched in”意思是“变得富含……”。

词义辨析

参考翻译

当冰川期开始后，大陆冰原会变大，持续地减少从海洋中蒸发并最终返回海里的水量。因为当水从海洋表面蒸发时，较重的同位素通常会被留下来，所以剩下的海水会逐渐变得富含氧18。

实战演练

…When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.

It can be inferred from the passage that precipitation formed from evaporated ocean water has

(A) the same isotopic ratio as ocean water.

(B) less oxygen 18 than does ocean water.

(D) a different isotopic composition than has precipitation formed from water on land.

(E) more oxygen 16 than has precipitation formed from fresh water.

答案：B

答案解析

题目问的是从海洋中蒸发的水形成的降水包含什么，从选项可推出题目问的应该是蒸发的海洋水的氧元素组成。根据“precipitation formed evaporated ocean water”的意思可以定位到“When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it”（海水蒸发后通过降雨的形式回到陆地），根据这一个句子我们无法直接获得答案，但是其后的“because”一句阐述了一个原理“当水从海洋表面蒸发时，较重的同位素通常会被留下来，所以剩下的海洋水包含的氧18会越来越多”（因为氧18比较重），由此可推断出“蒸发了的海洋水中包含的氧18比较少”，从而得出正确选项为B“（蒸发的海洋水）比海洋水含有更少的氧18”。

选项A“（蒸发的海洋水）和海洋水的氧元素比例相同”错误。从文中所给的信息，我们并不知道“ice contained in continental ice sheets”、“precipitation formed from water on land”和“precipitation formed from fresh water”的氧元素构成，因此排除选项C“（蒸发的海洋水）比大陆冰川的冰含有更少的氧18”、选项D“（蒸发的海洋水）的氧元素组成与从陆地水形成的降水是不同的”以及选项E“（蒸发的海洋水）比淡水形成的降水含有更多的氧16”。